emscripten: Fix the test run for Linux
emscripten: Fix the test run for Linux

- The test runs `python`, which exists on macOS as Python 2. On Linux,
  we don't have this luxury: Python 3 is called `python3`.
- The
  [emscripten software does support Python 3](https://emscripten.org/docs/building_from_source/toolchain_what_is_needed.html#emscripten-tools-and-dependencies),
  and this is where we should be moving given Python 2 is EoL in 2020, but
  let's fix it like this for now to get us moving as I thnk moving to
  Python 3 is a bigger piece of work.

Closes #15601.

Signed-off-by: Michka Popoff <7b0496f66f66ee22a38826c310c38b415671b832@gmail.com>

Linuxbrew/homebrew-core,rwhogg/homebrew-core,rwhogg/homebrew-core,maxim-belkin/homebrew-core,maxim-belkin/homebrew-core,LinuxbrewTestBot/homebrew-core,LinuxbrewTestBot/homebrew-core,Linuxbrew/homebrew-core